mime: take Content-Transfer-Encoding into account

Lync file transfer sends file metadata in XML format encoded as
quoted-printable. This has to be decoded prior processing.

Implemented for both libpurple and GMime backends.
purple: remove unused media stream parameters

"transport-protocols" and "demultiplex-func" are no longer used.
filetransfer: parse incoming Lync-style filetransfer invite
filetransfer: initialize media session for incoming data
filetransfer: send downloadFile request after establishing a connection
WIP: duplicate RTP candidates, and add extra SDP for ft/desktop sharing